Ever wondered why some lenses are a lot bigger, more expensive and often on paper less impressive than other lenses. For example why is one 70-200 mm lens over $2000 and a 55-300 lens can be as cheap as $400? The cheaper lens has a wider and longer range so it stands to reason it would be dearer.....right?
It all has to do with onions! Well "Onion Ring Theory" anyway.
Onion ring theory (to be honest, up front, not really a thing) states empirically that "each level of complexity (each onion ring out from the middle or base) added to a base formula multiplies the cost and other relevant factors of the item exponentially". Sounds logical, so it needed a name.
Lens designers must compromise with every lens design they make. Some lenses are designed to a price or have a specialised use rather than being more generically useful. Some lenses are "Pro" grade and others just good without being overly showy, or are simple, easily designed and often unchanged in design over long periods. Even if a lens is considered "no holds barred", it will come with other issues such as an extreme price and/or size.
For ease of writing, all lenses focal lengths will be full frame, so you will need to convert to your flavour of crop sensor equivalent.
The first onion ring is the humble 50mm f1.8 or the "nifty" fifty or sometimes the 40mm pancake lens (an old fashioned favourite making a comeback). Their original, more versatile use is a bit out of date on most smaller sensor SLR cameras as they are now a true portrait lens rather than an all rounder on a full frame camera. Always blisteringly sharp (even old ones and cheap ones!), they are so simple in design that size, price and weight are insignificant. Owning a 50mm f1.8 lens or the like gives the user two things, but takes away one. They are "fast" in maximum aperture, letting in up to 16 times more light down the barrel than a standard zoom and they can provide very shallow depth of field at the same time. What they loose is a zoom function (although they are a great tool for learning composition by moving your feet). The other type of lens in this class is the much maligned standard or "kit" zoom, including the most basic of short telephoto zooms. Optically stable if unexciting, many a modern image maker has kicked a goal or two or started a career with one of these simple wonders.
The second ring introduces the added complication of zooming or making the lens a bit wider or longer in focal length, faster in maximum aperture, wider in zoom range such as the 18-135 or maybe adding close focussing or macro. Sometimes a lens is just optically better. This usually adds substantially to the price, size and weight of the lens and sometimes compromises other features. For example, macro lenses usually only offer f2.8 as their maximum aperture, but in turn many fast portrait lenses have poor close focus capabilities. Some lenses add their own inherent benefits, such as zooms usually having better close focus capabilities than primes, but generally for the extra "onion ring" there is only one benefit offered.
The third ring is when the lens displays two of the above features. Usually not too crazy, these are the seriously good, but not unfeasibly difficult to make lenses such as the 85 or 28mm f1.8, F2.8 standard pro zooms or the moderately extended range wide angle and long tele zooms . Because the lens is now a premium price bracket and increasingly difficult to make, it is usually better constructed than its cheaper counterparts. Often the best value "semi pro" glass resides here and many brands such as Canon are making lenses such as the excellent 16-35 f4L IS as an option to the dearer 16-35 f2.8L IS.
The fourth ring is the realm of the super lenses. Here the lens can have three or four "rings" of features such as being super wide angle, fast aperture wide and long zooms, long and fast telephotos or just super fast/super sharp wide and short tele primes. Pro build tends to the norm, offering a level of robustness and often weather proofing that matches their high optical talents (and price). Thanks to never ending lens wars, top end zoom lenses have caught up with most regular primes in sharpness (but not maximum aperture and the best primes are still superior in consistency) and lenses unthought of 20 years ago are now readily available. There has even been, thanks to high pixel counts on camera sensors, a push to increase the optics of already good lenses beyond levels seen before. Lenses worthy of mention at this stage are all of the f2.8 pro zooms, the Zeiss and Sigma ART ranges and the very best, long, wide or fast primes from all camera makers.
Note; stabilisers were once considered an "onion ring" of benefit, but are now pretty standard in many lens designs. Makers who put their stabilisers in the camera do however offer this to all lenses where in lens stabilising is not included in many prime lenses.
The thing to remember here is that very good optics are often available in the first to third onion rings. The lesser offerings in many ranges often only compromise lens speed by about one or half of a stop and provide excellent optics in a small, cheap and light package. Some are even better than their dearer stable mates and often with fewer compromises common in more specialised lenses. An f1.8 lens on a Full frame camera will give you the same depth of field as an expensive f1.2 lens on a micro 43 camera and conversely f5.6 on a 17mm (acting as a 35mm in m43) has a lot more depth than f5.6 on a 35mm full frame camera, so also consider the format when working out your lens needs.
The Canon 85 f 1.8 for less than $500 is very sharp, focusses faster than the 85 f1.2L at $2500 and is tiny compared to its "door stopper" cousin. The same goes for the 50mm f1.8 at $149 against the Canon 50 f1.2L at over $2000- the f1.8 is often said to be as sharp or sharper at smaller apertures, but the faster lens has better "Bokeh" and more wow factor.
Some of the very best lenses can be found at reasonable prices also. Olympus and Panasonic are using the smaller M43 sensor size for a variety of reasons, but the first and most important is lens design. The Olympus 75 f1.8 (relatively easy to make well in any format) is equivalent to a 150mm f1.8 on a full frame camera (nearly impossible to make- 135 f1.8 or 200 f2 are the historical maximums). This allows Oly/Pan to create some very powerful lenses without having to break the laws of physics.
A sample kit based on the conventional wisdom for a working pro;
Canon 16-35 f2.8, 24-70 f2.8, 100 f2.8 macro and 70-200 f2.8 "L" series lenses. Cover super wide to short telephoto with a constant f2.8 aperture. Cost $8000 au. approx and weigh about 4kg.
Canon 17-40L f4L , 70-200 f4L, 85 f1.8 and/or 135 f2, 50 f1.8, 40 f2.8, and macro extension tubes or two element macro filter. Covers the range just as well and can be one or two stops faster where it is needed with a pancake lens for light street shooting. if used with a crop and full frame body combination it covers 17-300mm. Cost is $3000 au and weight (if all is carried) is 2kg roughly.
What you choose is of course up to you, but before going nuts and getting the top of the line lenses (people who own these lenses always crow loud and clear that they have the best, but you would to if you just spent $1000's on that wonder of engineering and had nothing to compare to).